1. Field of the Invention
The present invention relates generally to a servo control system and more particularly to a servo control system for controlling a capstan motor or a head motor of a video tape recording and/or reproducing apparatus (hereinafter referred as VTR) which requires extremely high stability and accuracy.
2. Description of the Prior Art
Hitherto the head motor of the VTR for a broadcasting purpose has been controlled by an analog servo control system. An embodiment of the known analog servo control system will be firstly explained. FIG. 1 shows such an embodiment which controls the head motor of the VTR is such a manner that a horizontal synchronizing signal in a video signal reproduced by the VTR is locked in phase with an external reference horizontal synchronizing signal during the reproducing operation. The servo control system shown in FIG. 1 comprises proportional, integral and differential control loops. In FIG. 1 a magnetic video tape 1 is continuously moved by a capstan (not shown) at a given speed in the longitudinal direction of the video tape 1.
The video signal to be reproduced has been recorded as frequency modulated signals on tracks of the video tape 1 which extend substantially at right angles with respect to the longitudinal direction of the video tape 1. The video signal (frequency modulated signal) is reproduced by video heads 2 provided on a head drum 3. The head drum 3 is driven by a head motor 4.
In order to control the rotation of the head motor 4, there is provided the integral control loop. This integral control loop comprises a signal deriving means 5 in the form of a rotary transformer or a brush for deriving the reproduced video signal from the video heads 2. The frequency modulated video signal thus derived is supplied to a demodulator 6. which demodulates the reproduced video signal and supplies the demodulated video signal to an output terminal 7. The demodulated video signal is further supplied to a horizontal synchronizing signal separator 8 which separates the horizontal synchronizing signal TAPE H from the reproduced video signal. The horizontal synchronizing signal TAPE H thus separated is supplied to a phase comparator 9, to which also is supplied a reference horizontal synchronizing signal REF. H. The reference horizontal synchronizing signal REF. H may be a horizontal synchronizing signal of a local station. The phase comparator 9 detects a phase difference between the reproduced horizontal synchronizing signal TAPE H and the reference horizontal synchronizing signal REF. H, and produces a signal (pulse signal) corresponding to said phase difference. This signal from the phase comparator 9 is applied through a phase compensating circuit 10 and a gain adjusting variable resistor 11 to a variable frequency oscillator 12 which serves as a frequency modulator. The output signal from the variable frequency oscillator 12 is supplied through a serrasoid phase modulator 13 to a three-phase divider 14. The output signals from the three-phase divider 14 are supplied via a motor driving amplifier 15 to the head motor 4. The above construction constitutes the integral control loop which can lock in phase the reproduced horizontal synchronizing signal TAPE H with the reference horizontal synchronizing signal REF. H.
As described above the servo control system shown in FIG. 1 further comprises the differential control loop. To constitute this differential control loop there is provided a detector 16 of a known construction for detecting a rotation frequency of the head motor 4. The signal detected by the detector 16 is supplied to a frequency discriminator 17 which detects a frequency deviation of the rotation frequency of the head motor 4 and produces a signal proportional to said frequency deviation. This signal is supplied through a gain adjusting variable resistor 18, a mixer 19 and a low pass filter 20 to the serrasoid phase modulator 13 as a modulating signal.
The servo control system shown in FIG. 1 further comprises the proportional control loop. For this purpose the output signal from the phase comparator 9 is supplied through a gain adjusting variable resistor 21 to the mixer 19. In the servo control system of FIG. 1 there is further provided a second differential control loop which is used in addition to the proportional control loop for further increasing the control accuracy. To constitute this second differential control loop the output signal from the phase comparator 9 is supplied via a gain adjusting variable resistor 22 and a phase compensating circuit 23 to the mixer 19.
In the above construction the phase compensating circuits 10 and 23 are analog circuit networks composed of circuit elements such as an inductance L, a capacitance C and a resistance R for compensating an amplitude and phase of the signal. As the phase compensating circuits 10 and 23 circuit networks shown in FIGS. 2a and 2b, respectively may be utilized. A circuit network illustrated in FIG. 2c may be used as the low pass filter 20.
The above mentioned control loops are constituted by the following components.